To provide protection from corrosion metal parts are for example galvanically coated with base metals such as for example zinc, nickel, chromium, aluminum, magnesium and alloys of the aforementioned, and the resistance to corrosion of the metal coating is further improved by production of a conversion layer, frequently a passivation layer. To produce a passivation layer the metal surfaces are in many cases treated with chromium (VI)-bearing solutions. However, because of the high toxicity and cancerogenity of chromium (VI)-compounds in recent times manufacturers have changed over to producing such conversion layers with chromium (III)-bearing solutions. In many cases large amounts of cobalt (II)-compounds are added to those chromium (III)-bearing treatment liquids, to increase the anti-corrosion action of the conversion layers produced. It will be noted however that those treatment solutions also represent a problem as both chromium (III) and also cobalt (II) are not completely safe in view of their toxicity and it is not possible to ensure on the basis of analytical problems that the conversion layers are free of chromium (VI). There is therefore an urgent need to produce completely chromium- and cobalt-free anti-corrosion layers.
Recent processes produce protective layers having a binder system on an organic or silicon-organic base, to which corrosion-inhibiting or layer-reinforcing additives based on molybdenum, tungsten, titanium, zirconium, vanadium and other metals are added.
U.S. Pat. No. 6,524,403 describes a chromium-free composition for improving the resistance to corrosion of zinc or zinc alloy surfaces, wherein the composition contains a source for titanium ions or titanates, an oxidising agent and fluorides as well as compounds of metals of group II and the composition is substantially free of silicates and silicon dioxide. In particular strontium is used as the metal of group II.
EP 0 760 401 discloses an anti-corrosion composition which contains an oxidising agent, a silicate and/or silicon dioxide and metal cations, selected from Ti, Zr, Ce, Sr, V, W and Mo, oxymetal anions thereof and/or fluorometal anions thereof.
The chromium-free anti-corrosion agents known hitherto suffer from the disadvantage that either they do not provide adequate anti-corrosion properties in respect of the corrosion layers or they are not sufficiently stable to be used in a continuous process, or both.